Such a method is known from the application DE 10 2014 100 653.4 which has published as U.S. 2016/0356661 A1.
Axle-counting systems are known in which axles travelling past a counting unit are detected by means of induction. The problem with said systems is that electromagnetic fields, for ex-ample from air conditioning units mounted on the train, can lead to interference and miscounting.
WO 2005/093971 A1 describes a railway monitoring system in which sensor fibers which are attached to the track are used for counting axles. By coupling light into the optical fibers (sensor fibers), light is emitted into the fiber Bragg grating, wavelengths which are within the filter bandwidth around the Bragg wavelength, being reflected. The Bragg wavelength is generally defined as λB=neff·2Λ=neff·λ, where neff is the effective refractive index and Λ is the grating period of the fiber Bragg grating. As a result of a force acting thereon, the sensor fiber and thus the fiber Bragg grating is stretched and the reflection and transmission wave-length of the fiber Bragg grating changes such that, on the basis of the stretching of the fiber Bragg grating, light of different wavelengths is reflected and can be transmitted to an evaluation and analysis unit. The individual fiber Bragg gratings are mounted at a distance of 2.5 m apart from one another. The individual fiber Bragg gratings have different Bragg wavelengths, wherein the difference of the Bragg wavelengths has to be greater than the change in Bragg wavelength of the corresponding fiber Bragg grating owing to loads.
Application DE 10 2014 100 653.4 describes a rail measurement system for measuring a mechanical variable, which acts on a rail, by means of fiber optic sensors. The fiber Bragg gratings used in this case are attached to the rails at an angle of from ±30° to ±60°, in particular of ±45°, relative to the neutral fiber. This offers the advantage that shear strains, which lead to a positive or negative stretching and do not extend parallel to the neutral fiber, are detected by the fiber optic sensor unit.
One disadvantage of the known arrangements is that the sensitivity is not sufficient in order to detect every axle reliably, meaning that the safety integrity level required for trains (SIL4) is not ensured since the signals that emerge from the measurement of the shear stress are not suitable for performing a threshold value evaluation.